The dual seal ring arrangement shown in FIG. 4 of the Mahoff patent advantageously provides a redundant seal whereby should one of the seal rings fail, the other will maintain a seal between the coupling parts.
In Mahoff's dual seal ring coupling, two pairs of seal ring-engaging formations are required, one for each seal ring, for torsionally deflecting the seal rings to their seal-establishing conditions as the coupling parts are axially drawn together. Each formation pair has one formation formed on the female coupling part and the other formation formed on the male coupling part. In practice, these formations are customarily machined on the coupling parts.
Aside from the extra expense involved in machining four seal ring-engaging formations on the coupling parts rather than just two, Mahoff's dual seal ring arrangement has two significant drawbacks.
First, the seal ring-engaging formations must accurately be located on the male and female coupling parts to ensure that both seal rings are torsionally deflected to the extent required for establishing a tight seal between the coupling parts. If one of the seal rings becomes fully deflected before the other is deflected to its seal-establishing condition, then only one seal ring will exert sufficient radial pressure for establishing a seal between the coupling parts.
Second, the seal rings used in the Mahoff arrangement are required to be of different diameters, thus giving rise to the inconvenience of stocking two different sizes of seal rings for the same coupling.
The present invention overcomes the foregoing drawbacks, but retains the advantage of the dual seal ring redundancy.